200 million years ago, before the supercontinent of Pangea split up, Virginia was located far from any ocean. The rocks of Virginia had been crunched by the collision of the North American and African plates. Island arcs (similar to Japan, Indonesia, or the Aleutian Islands today) had been smushed against the Virginia coastline on the Iapetus Ocean. The additions of those island arcs expanded Virginia - until the collision with the African plate.
Africa made Virginia smaller, initially. The African Plate compressed the sediments of the continental shelf on the North American Plate, converting the Iapetus Ocean beaches of Virginia into hard crystalline rocks. Today, those old sediments are the Piedmont "basement" rocks (exposed clearly at Great Falls on the Potomac River).
Over millions of years, the African plate kept pushing against the North American Plate, and Virginia was caught in the middle. Finally, the bedrock on the eastern edge of Virginia cracked under the pressure. Mountains as high as 20,000 feet were pushed up, in the "Alleghenian orogeny." (The modern Himalayas and the plateau of Tibet are being created as India crashes into the Eurasian Plate.)
In addition, slabs of eastern Virginia were thrust westward perhaps 40-60 miles.1 About 200 million years ago, the rocks of modern-day Shenandoah National Park were once located roughly where I-95 is today... until the collision with Africa shoved the Blue Ridge to the west. Rocks now exposed in the Shenandoah Valley and the rest of the Valley and Ridge Province were folded, crumpled, broken, and thrust on top of each other. In southwestern Virginia, Pine Mountain was also thrust westward 4-11 miles.2 The great squeezing by the continental collisions narrowed the width of Virginia by as much as 200 miles.3
That collision was part of the formation of the supercontinent of Pangea. All the continental crust clumped together, but that wide crustal plate began to trap all the heat rising up from the mantle. Pressure from the trapped heat, plus perhaps changing flow patterns of molten rock in the mantle, began to pull Pangea apart just as Rodinia had cracked up earlier. No supercontinent lasts forever...
As the African and North American plates pulled apart, the crust in the middle of Pangea stretched. As the land spread out, that crust thinned and thinned - and then it cracked. Lava oozed out in eruptions of "flood basalts" on the surface. The volcanism was short and intense, concentrated in just 600,000-5 million years.4 The Central Atlantic Magmatic Province (CAMP) was one of the largest flood basalts in the history of the earth, extruding massive amounts of subducted rock that may have been pushed deep in the Avalon orogeny and then erupted later in the early stages of the Triassic Period.5 The carbon dioxide released during the flood basalt eruptions may have been a factor in the extinctions that define the end of the Triassic Period and the start of the Jurassic Period. Ultimately, a "spreading ridge" developed, oozing magma with the chemical composition of Mid-Ocean Ridge Basalt - and the Atlantic Ocean was born, expanding into its current configuration over the last 200 million years.
Basins formed at cracks where the land dropped, from Connecticut to Alabama, during the Triassic and Jurassic Periods in the Mesozoic Era. Steven Spielberg highlighted the Jurassic Period when he titled his movie "Jurassic Park," but it is typical to refer to Triassic basins when discussing the geology of Virginia.
In Northern Virginia, drivers headed west on I-66/Route 29 may notice the Culpeper Basin west of Centreville. The valley stretches to the Bull Run Mountains (where the interstate highway goes through Thoroughfare Gap, on its way to the Shenandoah Valley). The change in elevation reflects differences in the bedrock. The sediments in the Triassic Basin west of Centreville are eroding faster than the metamorphic bedrock of the Piedmont formations to the east (between I-95 and Centreville), and faster than the metamorphic/volcanic formations of the Blue Ridge to the west. Differential erosion has, for the moment, excavated a shallow valley in which Leesburg, Manassas, and Culpeper are located.
As the sediments filled in the basins, dinosaurs were walking on the land and shallow lakes. Some of their tracks were fossilized in rocks that today are excavated at construction sites and in quarries in Loudoun, Prince William and Culpeper counties.6 (Connecticut established Dinosaur State Park to protect an extraordinary set of tracks discovered in the 1960's by an alert bulldozer operator, who was preparing a site for a state office building.)
The landscape of Virginia in the Mesozoic Era, during the Triassic/Jurassic Periods, may have looked like the Basin and Range province in Utah/Nevada or the Red Sea today:
 Basin and Range Province (Central Valley of California on left, Great Salt Lake in upper right-hand corner), where continental crust is stretching east-west and "mountain ranges look like rows of caterpillars marching northward"7 Source: NOAA National Geophysical Data Center, Natural Hazards Viewer |
 Red Sea, where a new ocean is forming as continental plates split apart Source: NASA Goddard Earth Sciences Data and Information Services Center, Science Focus: Ethiopia, Red Sea, and Nile River |
As the basins sank, erosion deposited up to 20-25,000 feet of sand and silt into the basin.8 The sandstones created from those sediments are known as "redbeds" due to the oxidized iron or rust that coats the sand grains. The grains in the sandstone bedrock were cemented together more tightly than the Aquia Sandstone on the Coastal Plain. The reddish/purple sandstones from Triassic basins oxidized after quarrying to become the "brownstones" used in many of the buildings in Brooklyn, plus in the Smithsonian "castle" on the National Mall in Washington DC and the Stone House at Manassas National Battlefield Park.
In addition, some harder rocks formed in the basins. In some places where the crust thinned, molten magma rose from the mantle up into the Triassic basin sediments to the surface and cooled into igneous rocks known as basalt and diabase. Where magma cut through the sedimentary layers, it formed "dikes," which are near-vertical pipes of igneous rock surrounded by flat-lying sediments.
Where the magma squeezed between flat layers of sediments, it shoved some sandstone/shale layers upwards slightly before the igneous rock cooled into "sills" of diabase rock. Sills cooled before reaching all the way to the surface, but may be exposed later through erosion or excavation. Today, the slower-to-erode igneous rock has created ridges within the Triassic basins, including the ridge to the west of Route 28 between Dulles International Airport-Route 7. The famous Cemetery Ridge and Little Round Top at Gettysburg battlefield in Pennsylvania were also created by diferential erosion of the igneous vs. sedimentary bedrock.9
Today, quarries excavate the hard diabase located in dikes and sills and crush the igneous rock for use in asphalt, concrete, and building foundations.
 contact between vertical igneous dike and horizontal sediments at Bull Run Quarry (Fairfax County) |
 outcrop of Triassic shale/sandstone sediments in Horsepen Run at Dulles airport |
1. Evans, M.A., 1989, The structural geometry and evolution of foreland thrust systems,
northern Virginia: Geological Society of America Bulletin, v. 101, p. 352–354, http://bulletin.geoscienceworld.org/cgi/content/abstract/101/3/339 (last checked June 27, 2011)
2. Kentucky Geological Survey, "Did You Know That Pine Moutain Has Moved?" http://www.uky.edu/KGS/education/pinemountain.htm (last checked June 27, 2011)
3. U.S. Geological Survey, "Geology of NYC Region - Valley and Ridge Province," http://3dparks.wr.usgs.gov/nyc/valleyandridge/valleyandridge.htm (last checked June 23, 2011)
4. Andrea Marzolia, Fred Jourdan, John H. Puffer, Tiberio Cuppone, Lawrence H. Tanner, Robert E. Weems, Hervé Bertrand, Simonetta Cirilli, Giuliano Bellieni and Angelo De Min, "Timing and duration of the Central Atlantic magmatic province in the Newark and Culpeper basins, eastern U.S.A.," in Lithos Volume 122, Issues 3-4, March 2011, Pages 175-188 http://www.sciencedirect.com/science/article/pii/S0024493710003567 (last checked June 27, 2011)
5. John H. Puffer, "A Reactivated Back-arc Source For CAMP Magma," pp. 151-162 in The Central Atlantic Magmatic Province: Insights From Fragments of Pangea, American Geophysical Union Geophysical Monograph 136, 2003
6. "Dino footprints found in Gainesville," Potomac News and Messenger," August 19, 2011 http://www2.insidenova.com/news/2011/aug/19/4/dino-footprints-found-gainesville-ar-1250297/ (last checked September 7, 2011)
7. National Park Service, Geologic provinces of the United States (last checked June 27, 2011)
8. Rodger T. Faill, "The early Mesozoic Birdsboro central Atlantic margin basin in the Mid-Atlantic region, eastern United States," Geological Society of America Bulletin, vol.115, no. 4, pp.406-421, 2003 http://bulletin.geoscienceworld.org/cgi/content/abstract/115/4/406 (last checked September 7, 2011)
9. Geology and the Gettysburg Campaign, Commonwealth of Pennsylvania, June 2006, http://www.dcnr.state.pa.us/topogeo/education/es5/es5.pdf (last checked September 7, 2011)
